In fluid distribution systems, it is often necessary to perform operations on the pipeline while the pipeline is under pressure. In this respect, fittings are welded to the pipeline or T's are connected to the pipeline and openings are tapped in the pipeline by use of tapping tool or drill inserted through a pressurized drilling machine. Additionally, pipeline stopping machines which also have a pressure chamber for manipulation of the elements being inserted or extracted, are used with these fittings and T's to insert a stopper into a pipe to temporarily stop flow through the pipe and then subsequently these stoppers are removed and closure plugs are inserted into the fitting so as to effectively close the opening made in the pipe for the particular operation mentioned above.
The closure plugs of the prior art have usually been a threaded plug member adapted to be threaded into the pipeline stopper fitting by a pipeline stopper inserting and extracting machine or the like. Usually the plug was provided with an equalizing valve which is opened when the closure plug is attached to the operating bar of the stopping machine since it is desirable that pressure on both sides of the closure plug be the same during insertion and removal of the closure plug from the fitting. While the closure plug which is threaded into the fitting has been the most widely used in recent years, various efforts have been made to modify such plugs so that it is not necessary to use threads with the same. One such effort has include cam elements, spring-urged outwardly of the closure plug member, the cam elements arranged to be received into a groove provided in a fitting. This closure plug assembly required rather complicated and expensive movable elements both on the fitting and the plug for it to function and in addition, it did not function as a principal closure plug for the fitting but rather as a secondary closure plug as a cover plate was bolted to the fitting after the closure plug had been inserted into the fitting for the purpose of making a preliminary seal for the fitting and the opening in the pipe.
In other prior art arrangements the closure plug was provided with spring-urged plates arranged to move outwardly into a groove when the closure plug was being inserted into a fitting. The plates used to hold the plug in place in the fitting required a rather complicated spring arrangement and operating means.